The present invention relates to a cardiac pacemaker device and method for physiologically controlling the stimulation rate of a heart within a body.
The human body is equipped to adapt to the increased need for cardiac output during exercise. If the heart is functioning properly, the nervous system increases the heart rate and reduces peripheral resistance in response to exercise. Typical cardiac pacemakers stimulate the heart at a fixed rate, and therefore cannot change in order to meet the increased need for cardiac output during exercise. Thus a person using a typical cardiac pacemaker is somewhat constrained in his daily activities.
The typical fixed-rate cardiac pacemaker produces electrical stimuli at a rate of approximately 72 beats per minute. The stimuli are applied to the heart through an electrode system in contact with the heart wall. Each time the electrical stimulus is delivered by the pacemaker and applied to the heart wall, a contraction of the myocardium results. The contraction pumps blood to the body.
From the original fixed-rate cardiac pacemaker evolved the demand pacemaker. The demand pacemaker ceases to produce a stimulus when a spontaneous heart beat is detected. The presence of a spontaneous heart beat is indicated by a normal QRS complex in the electro- cardiogram. Patients with intermittent conduction in the heart benefit from the demand pacemaker because this pacemaker does not compete with normal beats of the heart when and if they occur. Several modifications to the basic demand pacemaker have been made which attempt to better approximate a body's normal response. One such modified pacemaker allows the choice of two stimulation rates. Stimuli are produced at a nominal rate when the pacemaker is acting in the fixed-rate mode and at a lower rate when intermittent spontaneous heart beats are detected, thus allowing more time for the heart to beat spontaneously.
In addition to sensing the presence of electrical activity in the ventricle, sensing of atrial activity has also been used. A person may have normal atrial rhythm and some form of atrial-ventricular block. Such a person would benefit from an atrial-sensing pacemaker that delivers a stimulus to the ventricles after each atrial excitation. The atrial pacemaker has seen limited use because ventricular conduction problems often precipitate atrial tachycardia or fibrillation rendering the atrial-sensed pacemaker inactive.
In an effort to increase the efficiency of cardiac pacing, certain pacemakers use sensing of the electrogram in the atria and in the ventricles to allow atrial-ventricular synchrony to be restored. To produce each heart beat first the atria are stimulated. After a delay, nominally equal to the normal A-V node delay, the ventricles are stimulated. The addition of the atrial contraction produces the normal added filling of the ventricle that comes with synchronous atrial systole. Such a pacemaker, called the A-V sequential pacemaker, increases the end diastolic volume and therefore, the stroke volume, causing additional blood flow. In addition A-V pacemakers incorporate dual sensing so that if the atria or ventricles beat, the spontaneously contracting chamber is not stimulated. More recently, a pacemaker with the combination of atrial and ventricular sensing, atrial and ventricular pacing, and atrial and ventricular inhibit has been developed. This pacemaker is commonly called a "DDD" pacemaker.
Pacemakers using each of the above modes may also be programmed after implantation. Each patient has slightly different requirements to optimally stimulate the heart. Therefore, if certain parameters of the pacemaker can be changed after implant, the pacemaker may be "individualized" to the patient's needs. Parameters that are programmable after implantation may include the rate, stimulus voltage, stimulus current, and electrogram sensitivity. In the case of the A-V sequential pacemaker, there is sensing and stimuli for both the atria and the ventricles that may be programmable in addition to the A-V delay time.
Since the first fixed-rate pacemaker, there have been many advances, but even with programmability the pacemaker does not adequately and reliably sense the body's need for more blood flow during exercise. Physiological pacemakers based on the electrical activity of the heart have not been applicable to a major portion of the pacemaker-using population. A patient with a partially denervated heart requires sensing of body cardiac output needs from a source other than electrical activity within the heart. There are many physiological indicators available from which the need for increased cardiac output may be sensed. In an attempt to provide sensing information, the nerves leading to the heart, in particular the sympathetic nerves, will provide information processed by the brain that naturally increases the heart rate. Unfortunately, current technology prohibits the use of a long term nerve impulse transducer.
The pH of the blood also has been measured and used to control the rate of a cardiac pacemaker. PH transducers that are implantable for long periods of time are however difficult to produce and therefore are not yet in common use.
The following list of references disclose devices which may have some general relevance to the present invention: invention:
______________________________________ Reference Inventor ______________________________________ U.S. Pat. No. 3,867,950 Fischell U.S. Pat. No. 3,656,487 Gobeli U.S. Pat. No. 3,593,718 Krasner U.S. Pat. No. 3,828,371 Purdy U.S. Pat. No. 4,181,133 Kolenik et al. U.S. Pat. No. 4,023,121 Alley, III U.S. Pat. No. 4,228,803 Rickards U.S. Pat. No. 4,201,219 Bozal Gonzalez U.S. Pat. No. 4,202,339 Wirtzfeld et al. ______________________________________
Journal of Association for Advancement of Medical Instrumentation, "A Physiologically Controlled Cardiac Pacemaker", Krasner; Voukydis; and Nardella, Nov.-Dec. 1966, pages 14-16, and 20.
U.S. Pat. No. 3,867,950 to Fischell discloses a fixed rate rechargeable cardiac pacemaker which utilizes as its power source a single rechargeable cell battery which is recharged through the patient's skin by magnetic induction. The electronic pulse generating circuitry is designed such that the output pulse rate varies as a function of the battery voltage and also as a function of the body temperature. This device is different from the present invention in that the heart stimulation rate is controlled by ambient body temperature, in other words, a body core temperature which represents the average body temperature. Thus, ambient body temperature is measured by a charging capacitor having a high temperature coefficient located within the pulse generator circuitry. Since ambient body temperature does not vary appropriately as a function of muscle exertion, this device will not respond to a body's need for increased cardiac output due to muscular exertion.
U.S. Pat. No. 3,656,487 to Gobeli discloses an electronic demand heart pacemaker with different pacing and standby rates. The device stimulates heart rate at a first frequency in the continued absence of natural heart beats but allows the heart to beat naturally at any rate above a second lower standby frequency. The device reverts to a third frequency in a non-demand type operation in the presence of an interfering electrical noise pattern.
U.S. Pat. No. 3,593,718 to Krasner discloses a physiologically controlled cardiac pacer which uses respiratory rate to vary the production of electronic pulses which are fed to a constant current source connected to the ventricle. In another variation, pulses are fed to two separate constant current sources, one connected to the atrium and the other, with delay, to the ventricle.
U.S. Pat. No. 3,828,371 to purdy discloses a self-contained artificial heart which is adapted to vary the pulse rate and the stroke length in response to blood pressure. Variations in blood pressure are detected by means of a pressure sensitive transistor, thereby varying the rate of pumping of blood in response to blood pressure.
U.S. Pat. No. 4,181,133 to Kolenik et al. discloses a programmable tachycardia pacer which performs the dual functions of demand pacing as well as standby tachycardia break up. A command parameter control is used for programmably controlling the parameters of the pacer operation as well as of the tachycardia recognition and response.
U.S. Pat. No. 4,023,121 to Alley discloses an oscillator for a demand heart pacer which produces stimulation pulses in an inhibit mode at a first frequency and stimulation pulses at a second fixed rate mode at a second frequency. The device additionally employs an oscillator inhibitor which operates to define a variable second half of a refractory period to distinguish noise at a frequency approximately twice that of an acceptable heart frequency.
U.S. Pat. No. 4,228,803 to Rickards discloses a physiologically adaptive cardiac pacemaker having a demand generator which is sensitive to the electrical activity of the heart. The interval between a pacing stimulus delivered by a pacemaker generator and the evoked ventricular repolarization sets the escape interval of the generator for the subsequent stimulus, and thus physiologically controls the heart rate.
U.S. Pat. No. 4,201,219 to Bozal Gonzalez discloses a cardiac-pacemaker in which the generation of stimulation pulses is controlled by regulation signals detected in the nerves via receivers.
U.S. Pat. No. 4,202,339 to Wirtzfeld et al. discloses a cardiac pacemaker which measures the oxygen saturation of the blood as a control variable for influencing the frequency of stimulation. Determination of the blood oxygen saturation is made via light conductor probes implanted in the heart.
The publication entitled "A Physiologically Controlled Cardiac Pacemaker" by Krasner et al. discloses a cardiac pacemaker similar to that disclosed in U.S. Pat. No. 3,593,718 to Krasner, which uses respiratory rate to vary the production of electronic pulses to the heart.
One physiological indicator which responds directly to the need for higher cardiac output due to exercise but which is not disclosed in any of the above references, is venous blood temperature. When a person exercises, the muscles do work. Since the muscles are not completely efficient, they also produce heat. It is the task of the blood stream to dispose of the additional heat produced by the exercising muscles.
The heat produced by a muscle raises the temperature of the blood leaving it. This blood returns directly to the right heart before passing through heat dissipating tissue. The blood returning to the right heart therefore contains information about the work output of the body musculature. Right ventricular blood is a combination of blood from the upper body via the superior vena cava and from the lower body via the inferior vena cava. The blood from these two areas of the body is mixed in the right atrium and again in the right ventricle where it is joined with blood drained from the heart. Thus, right ventricular blood temperature is the average temperature returning from all the body and so reflects the average work output of the body. Since the heat causing the temperature rise during exercise is generated by exercising muscles, the right ventricular blood temperature reflects the average level of exercise and, hence, the need for increased cardiac output or increased heart rate.
Accordingly, it is an object of the present invention to provide an improved cardiac pacemaker in which stimulation of the heart is varied in accordance with the level of muscular exertion.
This and other objects and advantages of the present invention will become more apparent in the following figures and detailed description.